During a cold start of an internal combustion engine, or in a period immediately after the cold start thereof, the temperature of intake valves and that of the walls of the intake ports are low, and fuel adhered to these parts is not likely to atomize. As a result, the amount of fuel flowing into the combustion chamber is small with respect to the amount of fuel injected from the fuel injectors.
The fuel adhered to the intake valves or the walls of the intake ports forms so called a wall flow. It is known that the wall flow flows into the combustion chamber at a delay with respect to the flow of fuel which is directly aspirated into the combustion chamber in the form of mist or vapor.
During the engine start and a period immediately after the engine start, the fuel injection amount is generally increased to compensate for the fact that a part of the injected fuel forms a wall flow. When the fuel is constituted by heavy components, the rate of atomization is low in comparison with a fuel constituted by light components, and hence, the fuel easily forms a wall flow. In order to accomplish a successful engine start without suffering a shortage of fuel in the combustion chamber irrespective of the fuel mass, i.e., heavy or light, an increase in the fuel injection amount is determined on the assumption that the fuel is constituted by heavy components.
When a fuel constituted by light components is used, the wall flow decreases because the fuel is more likely to atomize than a fuel constituted by heavy components, and therefore, the fuel amount supplied to the combustion chamber is apt to be excessive. Accordingly, the air/fuel ratio of the air-fuel mixture in the combustion chamber becomes rich, which increases the emission of unburned hydrocarbon (HC).
JP2003-003872A published by the Japan Patent Office in 2003, proposes to vary the operation characteristics of the intake valve to cope with this problem. This prior art relates to an engine provided with a variable valve timing control (VTC) mechanism which enables variation in the opening/closing timing of the intake valve, and a variable valve event and lift control (VEL) mechanism which enables variation in the lift amount of the intake valve. The prior art proposes to decrease the lift amount of the intake valve and retard the opening/closing timing thereof during a period from the engine start until a certain time point when the engine is warming up.
When the lift amount of the intake valve is reduced, the flow area of the intake valve is also reduced, and the velocity of the intake air aspirated through the intake valve increases. The increased velocity of the intake air promotes atomization of the fuel adhered to the intake valve and the walls of the intake port as well as to accelerate the fuel flow towards the combustion chamber. As a result, the amount of wall flow decreases.
This phenomenon reduces wall flow equally irrespective of differences in the components of the fuel. According to the prior art, therefore, it is possible to decrease the increase in the fuel injection amount required for engine start and, as a result, the increase required for fuel constituted by heavy components and the increase required for the fuel constituted by light components are almost the same such that the engine can be started with an identical increase irrespective of the fuel characteristics while suppressing increase in the emission of unburned HC.
By increasing the intake air velocity, mixing of air and fuel in the combustion chamber is enhanced, and stable combustion is ensured even when the ignition timing is retarded. Retarding the ignition timing brings about a high exhaust gas temperature which enables early activation of an exhaust gas purification catalyst.
The prior art therefore has an effect of improving the exhaust gas composition during engine start-up as well as accelerating a temperature increase in the exhaust gas.